The present invention relates to a magnetic recording medium with excellent electromagnetic transfer characteristics and high durability, whereby the medium comprises a magnetic layer having ferromagnetic powder and a binder dispersed in it, and the magnetic layer is provided on a non-magnetic support, and in particular to a magnetic recording medium suitable for high-speed rotating disk.
The magnetic recording medium is widely used in such applications as recording tape, video tape, floppy disk, etc. The magnetic recording medium comprises a magnetic layer having ferromagnetic powder and a binder dispersed in it, and the magnetic layer is laminated on a non-magnetic support.
The magnetic recording medium must be at high level in such characteristics as electromagnetic transfer characteristics, running durability, and running performance. Specifically, an audio tape for reproducing musical sound must have an ability to reproduce original sound with higher quality. A video tape must have high electromagnetic transfer characteristics such as high ability to reproduce original image or picture.
Together with the requirements to have high electromagnetic transfer characteristics, the magnetic recording medium must have good running durability as described above. To ensure good running durability, an abrasive material and a lubricant are generally added in the magnetic layer.
However, in order to ensure good running durability by the use of an abrasive material, it is necessary to increase the adding quantity of the abrasive material to some extent, and this leads to the decrease of filling ratio of ferromagnetic material. Also, in case an abrasive material having large particle size is used to ensure good running durability, the abrasive material is very likely to protrude from the surface of the magnetic layer. Therefore, the improvement of the running durability using the abrasive material often leads to deterioration of the electromagnetic transfer characteristics.
In order to improve the running durability by the use of a lubricant, it is necessary to increase the adding quantity of the lubricant. As a result, the binder becomes easily plasticized, or durability of the magnetic layer may be decreased, or pasting or sticking may occur when large quantity of lubricant is used.
JP-A-59-65931 describes a magnetic recording medium using a lubricant, which comprises a diester of neopentyl-glycol, trimethylol propane, and pentaerythritol with fatty acid and ethylene oxide denatured ester. When this is applied in a tape for high recording density as currently used, repeated running durability is not satisfactory and such problems as head contamination, output decrease, etc. arise.
JP-B-66-18063 describes a diester of carboxylic acid and dihydric alcohol as lubricant, and JP-B-72-14648 describes a diester of dicarboxylic acid. JP-A-56-80829 describes an aliphatic dibasic acid ester, and JP-A-59-28236 describes a substance, which comprises dibasic acid ester having a specific chemical structure and a graphite with a specific average particle size. Further, JP-A-59-186129 describes a magnetic recording medium using dioleyl maleate.
However, in all of the substances of the above inventions, storage stability and resistance to hydrolysis are low, and durability is decreased during long-term storage under high temperature and high humidity conditions. In the application for a tape with high recording density, repeated running durability is not satisfactory.
JP-A-61-139921 describes a magnetic recording medium having a magnetic layer, which contains a fatty acid ester of polyhydric alcohol and phosphoric acid ester of phenylpolyethylene glycol. However, its dispersion property is not satisfactory. Polyethylene glycol portion is too soft and unsatisfactory in strength and durability. Ethylene glycol portion is low in thermal stability and long-term storage property.
JP-A-7-73450 describes a magnetic recording medium having a magnetic layer, which comprises a dibasic acid ester with a specific chemical structure as lubricant. However, its durability is not satisfactory and storage stability is low. Also, the binder is easily plasticized, strength is decreased, and calender roll is easily contaminated.
JP-A-7-192250 describes a magnetic recording medium, which comprises a diester containing dihydric alcohol and unsaturated fatty acid. As examples, description is given on simultaneous use of dioleate of neopentyl glycol with stearic acid and butyl stearate. However, it is not satisfactory in durability and storage property, and it is disadvantageous in that calender roll is likely to be contaminated in the manufacturing process.
It is an object of the present invention to provide a magnetic recording medium having high repeated running durability, and also to provide a magnetic recording medium, which has high running durability at low temperature and good storage stability under high temperature and high humidity conditions, has roll contamination unlikely to occur in calender process, exhibits high productivity and has high dispersion property, dispersion stability and high electromagnetic transfer characteristics.